Electromagnetic wave transmission systems including a dielectric window for transmitting high-frequency highpower electromagnetic energy to a load from a source of suchenergy such as a resonant cavity of a klystron



DIELECTRIC March 14, 1967 o HEIL,

ELECTROMAGNETIC WAVE TRANSMISSTON SYSTEMS lNCLUDING A WINDOW FOR TRANSMITTING HIGH-FREQUENCY HIGH-POWER ELECTROMAGNETIC ENERGY To A LOAD FROM A SOURCE OF SUCH ENERGY SUCH AS A RESONANT CAVITY OF A KLYSTRON Filed 001, 7, 1963 2 Sheets-Sheet 1 v vAcuqM j 5 El! INVENTOR.

OSKAR HEIL AT TORNE Y March 14, 1967 o. HElL 3,309,558

ELECTROMAGNETIC WAVE TRANSMISSION SYSTEMS INCLUDING A DIELECTRIC WINDOW FOR TRANSMITTING HIGH-FREQUENCY HIGH-POWER ELECTROMAGNETIC ENERGY TO A LOAD FROM A SOURCE OF SUCH ENERGY SUCH AS A RESONANT CAVITY OF A KLYSTRON Filsd Oct. 7, 1963 2 Sheets-Sheet 2' 26 Ezzzm l J Li; I 1 LI 8 .17 2 PRIOR ART 42 VACUUM I INVENTOR.

4-53.74 BY M /%Zi% ATTORNEY United States Patent Ofitice Patented Mar. 14, 1967 ELECTROMAGNETIC WAVE TRANSMISSION SYS- TEMS INCLUDING A DIELECTRIC WINDOW FOR TRANSMITTING HIGH-FREQUENCY HIGH- POWER ELECTROMAGNETIC ENERGY TO A LOAD FROM A SOURCE OF SUCH ENERGY SUCH AS A RESONANT CAVITY OF A KLY- STRON Oskar Heil, San Mateo, Calif., assignor, by mesne assignments, to Varian Associates, a corporation of California Filed Oct. 7, 1963, Ser. No. 314,183 8 Claims. (Cl. 3l55.39)

This invention relates generally to high frequency electromagnetic wave transmission systems having windows therein transparent to electromagnetic wave energy, and particularly to high frequency electromagnetic wave transmission systems designed to reduce secondary electron bombardment on the surface of the windows located therein.

The windows of the type under consideration are particularly useful in the input and output waveguide connections of an electron tube which operates at microwave frequencies. In such applications electromagnetic energy must be transmitted between the evacuated interior of the tube envelope and exterior waveguide systems which may be maintained at atmospheric or other pressures. Other examples of such windows are found in applications where a seal is required to prevent the escape of gas from a pressurized system or where a seal is required to contain a cooling or insulating fluid in a waveguide system. Microwave windows have taken various forms using a variety of metal and dielectric materials.

The use of dielectric microwave windows introduces a number of electrical problems. The electrical problems include voltage breakdown in the presence of high electric and magnetic fields in the area of the dielectric window and electron multipactor elfects on the surface of the dielectric window.

Dielectric output windows transparent to electromagnetic energy have a coefficient of secondary electron emission greater than unity when subjected to bombardment by electrons whose energy levels lie within a certain range. This secondary electron emission coelficient characteristic renders the dielectric window susceptible to destructive heating due to single surface multipactor which occurs when the dielectric window is exposed to high frequency electromagnetic fields. This phenomenon is explained at length in an article appearing in the IRE Transactions of the Professional Group of Electron Devices, volumes ED8, Number 4, dated July 1961.

It is the object of this invention to provide improved high frequency electromagnetic wave transmission systems designed to shape electromagnetic fields in the vicinity of windows mounted therein so as to reduce secondary electron bombardment or electron multipactor on the surfaces of the windows and thereby raise the power handling capacity of the systems.

Briefly described, this invention relates to electromagnetic wave transmission systems containing standing wave energies and dielectric windows mounted therein transparent to the electromagnetic wave energies flowing through the systems. One electromagnetic wave transmission system contains a standing wave section having alternate regions of low and high electric field intensities distributed throughout the section. A dielectric window transparent to the standing wave energy is located in one of the low intensity electri-c field regions of the section. The window is located in the low intensity electric field region of the section that is between the high intensity electric field region of the section and the portion of the section closest to the original source of the standing wave energy.

In the drawing:

FIGURE 1 is an elevational view of a klystron tube incorporating a radio-frequency output window wherein the resonant cavity associated with the output coupling arrangement is shown in vertical section;

FIGURE 2 is an enlarged sectional view of a prior art output coupling arrangement;

FIGURE 3 is an enlarged sectional view of the output coupling arrangement of FIGURE 1;

FIGURE 4 is a diagrammatical representation of standing wave energy in a waveguide indicating the position of the window with respect to the lines of force of the electric field of the standing wave;

FIGURE 5 is a cross-sectional view of another embodiment of an output coupling arrangement in accordance with this invention; and

FIGURE 6 is a cross-sectional view of a further embodiment of an output coupling arrangement in accordance with this invention.

Referring to the figures, FIGURE 1 shows a klystron which includes an electron gun section 4, a radio-frequency interaction section 6, and a collector section 7. As is well known in the art, the electron gun section, the

radio-frequency section, and the collector section are hermetically united in axial alignment to enable the projection of an electron beam through a series of drift tube sections 8, each terminating within a cavity in a conically tapered end portion 9 spaced from the associated end of an adjacent drift tube section to provide an interaction gap 12 therebetween. Drift tube sections 8 are supported in axially spaced alignment by relatively heavy transversely extending annular metallic plates 13, preferably fabricated from oxygen-free high-conductivity copper. Tuners 14 are provided in each of the first two cavities in the interaction section 6. Input cavity 15 contains an input loop 16 part of which is shown in FIGURE 1. Output cavity 17 is provided with an output coupling arrangement 18 which comprises a first rectangular waveguide section 19 coupled to the output cavity 17.

An output window 20 is asymmetrically positioned in a pill box or disk shaped waveguide section 22 in the output coupling arrangement 18. The disk shaped waveguide portion 22 is located between a second rectangular waveguide section 24 and the first rectangular waveguide section 19. A flange 25 is provided on the terminal end of the second rectangular waveguide section 24 so as to permit coupling of the electromagnetic energy from the klystron to a load.

FIGURE 2 is an illustration of the prior art wherein the output window 26 is symmetrically positioned in the disk shaped waveguide portion 27 located between two rectangular waveguide portions 23 and 29, respectively. One of the rectangular waveguide portions 28 is connected up to an output resonant cavity part of which is shown.

FIGURE 3 is an enlarged sectional view of the output coupling arrangement of FIGURE 1 and illustrates the principle of the invention. The output window 20 of FIGURE 3 is asymmetrically positioned in the disk shaped waveguide section 22 of the output coupling arrangement 18.

Electromagnetic wave energy flowing through the output coupling arrangement 18 sets up standing wave energy in the disk shaped waveguide section 22 due to reflections of a portion of the working electromagnetic wave energy between side walls 23 of the disk shaped waveguide section 22. The standing wave energy thus created in the disk shaped waveguide section 22 has alternate low and high intensity electric fields distributed throughout the disk shaped Waveguide section 22. The dielectric window is located in a low intensity electric field region that is between the center plane of the disk shaped waveguide section 22 perpendicular to the longitudinal axis of the disk shaped waveguide section and the portion of the disk shaped waveguide section 22 adjacent to the first rectangular waveguide section 19 Placing the dielectric window 20 in the low intensity electric field region closest to the first rectangular waveguide section 19 permits the secondary electrons released from the surface of the window 20 to travel away from the window 20.

FIGURE 4 diagrammatically illustrates the lines of force of the electric field of the standing wave in a disk shaped waveguide. A waveguide window is positioned in a low intensity electric field region in accordance with this invention so that secondary electrons represented by the arrow 42 are accelerated in a direction away from the window. Location of the Window in either the center portion, which is the high intensity electric field region, of the waveguide 44 of FIGURE 4 or in the other low intensity electric field region of the waveguide 44 will not suffice since in both cases the high intenisty electric field causes secondary electrons released from the surface of the window to return to the surface of the window 40 to enhance electron multipactor.

FIGURE 5 illustrates another embodiment of an output coupling arrangement wherein dielectric window is located in a disk shaped waveguide section 52 which is connected to two rectangular waveguide sections. A loop 54 is provided in the disk shaped waveguide section 52 for the purpose of supplying pumping electromagnetic wave energy having a plane of polarization perpendicular to the plane of polarization of the working electromagnetic wave energy fiowing through the output coupling arrang ment. The pumping electromagnetic wave energy that is supplied to the disk shaped waveguide section 52 by means of the loop or probe 54can have a frequency equal to or higher or lower than the frequency of the working electromagnetic wave energy flowing through the output coupling arrangement. However, the frequency of the pumping electromagnetic wave energy must be below the cutoff frequency of the disk shaped waveguide section 52. Since the pumping electromagnetic wave energy has its plane of polarization perpendicular to the working electromagnetic wave energy, the pumping electromagnetic wave energy does not interfere with the working electromagnetic wave energy flowing through the output coupling arrangement. The standing wave energy set up in the disk shaped waveguide section 52 by both the pumping electromagnetic wave energy introduced by the probe 54 or loop and the working electromagnetic wave energy contains regions of low and high electric field intensities. The window 50 is asymmetrically positioned in the disk shaped waveguide section 52 as in FIGURES 1 and 3 and is situated in the low intensity electric field region between the center plane perpendicular to the longitudinal axis of the disk shaped waveguide section 52 and the rectangular waveguide section connected to the output cavity of a klystron. The pumping electromagnetic wave energy supplied to the disk shaped waveguide section 52 by the probe or loop 54 is furnished by connecting the probe or loop 54 to an electron tube device, such as a klystron.

FIGURE 6 discloses another embodiment of an output coupling arrangement somewhat similar to the operation of the output coupling arrangement of FIGURE 5. A window is asymmetrically positioned in a disk shaped waveguide section 62 which is connected between two rectangular waveguide sections. The electromagnetic wave energy transmitted through the output coupling arrangement sets up standing wave energy in the disk shaped waveguide section 62 as in FIGURES l and 3. The window 60 is located in the low intensity electric 4 field region closest to the rectangular waveguide section connected to the output cavity of a klystron.

A magnetic field having lines of force perpendicular to the surface of the window 60 is provided by means of, for example, a pair of permanent magnets 63 and 64. The magnetic field set up by magnets 63 and 64 has a value smaller than the gyromagnetic resonance field and can be as weak as a field of about 50 gauss. However,

the magnetic field set up by the magnets 63 and 64 does not afiect the electromagnetic wave energy being transmitted through the output coupling ararngement.

In this arrangement, free electrons in front of the window 60 which are normally forced to oscillate in the plane of polarization of the working field have the directions of their oscillations rotated by the magnetic field into a plane of polarization perpendicular to theplane of polarization of the working electromagnetic wave energy being transmited through the output coupling arrangement. Hence, pumping electromagnetic wave energy is set up in the disk shaped waveguide section 62 having the same frequency as the frequency of the working electromagnetic wave energy transmitted through the output coupling arrangement.

As in the output coupling arrangement of FIGURE 5, electrons in front of the window 60 will be pumped away by the inhomogenons fields set up by both the pumping and working electromagnetic waves.

It is to be understood that the above described arr-angements are simply illustrative of the application of the principles of the invention. Numerous other arrangements may be readily devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

I claim:

'1. An electromagnetic wave transmission system comprising, in combination, a first rectangular waveguide section adapted to pass electromagnetic wave energy received from a source, a disk shaped waveguide section connected to said first rectangular waveguide section and adapted to transmit electromagnetic wave energy received from said first rectangular waveguide section, and a second rectangular waveguide section connected to said disk shaped waveguide section and adapted to transmit electromagnetic wave energy received from said disk shaped waveguide section, said disk shaped waveguide section containing standing wave energy having alternate regions of low and high electric field intensities, a dielectric window transparent to electromagnetic wave energy transversely mounted in said disk shaped waveguide section, said dielectric window being located in the low intensity electric field region of said disk shaped waveguide section between the 'high intensity electric field region of said disk shaped waveguide section and said first rectangular waveguide section.

2. Claim 1 wherein said disk-shaped waveguide section is provided with a loop for introducing pumping electromagnetic wave energy into said disk-shaped waveguide section, said pumping electromagnetic wave energy having a plane of polarization perpendicular to the electromagnetic wave energy transmitted through said electromagnetic wave transmission system, whereby secondary electrons emitted from the surface of said dielectric window are driven away from said window due to the low intensity electric field region provided in front of said window which is set up by the standing wave energy derived from both the electromagnetic wave energy transmitted through said disk-shaped waveguide section and said pumping electromagnetic wave energy supplied to said disk-shaped waveguide section.

3. Claim 1 having means for providing a magnetic field having lines of force substantially perpendicular to the surfaces of said dielectric window to rotate the oscillations of secondary electrons in front of said dielectric window and thereby form a pumping electromagnetic wave having a plane of polarization perpendicular to the plane of polarization of the electromagnetic wave energy transmitted through said electromagnetic wave transmission system, whereby secondary electrons emitted from the surface of said dielectric window are driven away from said window due to the low intensity electric field region provided in front of said window which is set up by the standing wave energy derived from both the electromagnetic wave energy transmitted through said disk-' shaped Waveguide section and said pumping electromagnetic wave energy formed in said disk-shaped waveguide section.

4. A klystron including an electron gun section, a radiofrequency interaction section and a collector section defining an evacuated envelope, said radio-frequency interaction section comprising a plurality of axially aligned resonant cavity portions, each resonant cavity portion con tains electromagnetic wave energy and includes an interaction gap and conductive metallic surfaces within the evacuated envelope, and an electromagnetic wave transmission system associated with at least one of said resonant cavity portions, said electromagnetic wave transmission system comprising a waveguide section containing standing wave energy having alternate regions of low and high intensity electric fields, said waveguide section being provided with a dielectric window transparent to. electromagnetic wave energy mounted therein, said-window being located in the low intensity electric field region within said electromagnetic wave. transmission system between the high intensity electric field region in said waveguide section and said associated resonant cavity.

5. A klystron including an electron gun section, a radiofrequency interaction section and a collector section defining an evacuated envelope, said radio-frequency interaction section comprising a plurality of axially aligned resonant cavity portions, each resonant cavity portion con tains electromagnetic wave energy and includes an interaction gap and conductive metallic surfaces within the evacuated envelope, and an output coupling arrangement associated with at least one of said resonant cavity portions, said output coupling arrangement comprising a first rectangular waveguide section adapted to transmit the electromagnetic wave energy received from said associated resonant cavity, a disk shaped waveguide section connected to said first rectangular waveguide section and adapted to transmit electromagnetic wave energy received from said first rectangular waveguide section, and a second rectangular waveguide section connected to said disk shaped waveguide section and adapted to transmit electromagnetic wave energy received from said disk shaped waveguide section, said disk shaped waveguide section containing standing Wave energy having alternate regions of low and high electricfield intensities, a dielectric window transparent to electromagnetic wave energy transversely mounted in said disk shaped waveguide section, said dielectric window being located in the low intensity electric field region of said disk shaped waveguide section between the high intensity electric field region of said disk shaped waveguide section and said first rectangular wave- 9 guide section.

6. A klystron including an electron gun section, a radiofrequency interaction section and a collector section defining an evacuated envelope, said radio-frequency interaction section comprising a plurality of axially aligned resonant cavity portions, each resonant cavity portion contains electromagnetic wave energy and includes an interaction gap and conductive metallic surfaces within the evacuated envelope, and an output coupling arrangement associated with at least one of said resonant cavity portions, said output coupling arrangement comprising a first 6 rectangular waveguide section adapted to transmit the electromagnetic wave energy received from said associated resonant cavity, a disk shaped waveguide section connected to said first rectangular waveguide section and adapted to transmit electromagnetic wave energy received from said first rectangular waveguide section, and a second rectangular waveguide section connected to said disk shaped waveguide section and adapted to transmit electromagnetic Wave energy received from said disk shaped waveguide section, said disk shaped waveguide section containing standing wave energy having alternate regions of low and high electric field intensities, a dielectric window transparent to electromagnetic wave energy transversely mounted in said'disk shape-d waveguide section, said dielectric window being asymmetrically positioned in said disk shaped waveguide section and located in the low intensity electric field region between the center plane perpendicular to the longitudinal axis of said disk shaped waveguide portion and said first rectangular waveguide section.

7. Claim 6 wherein said disk-shaped waveguide section is provided with a loop for introducing pumping electromagnetic wave energy into said disk-shaped waveguide section, said pumping electromagnetic wave energy having a plane of polarization perpendicular to the electromagnetic wave energy transmitted through said output coupling arrangement, whereby secondary electrons emitted from the surface of said dielectric window are driven from said window due to the low intensity electric field region provided in front of said window which is set up by the standing wave energy derived from both the electromagnetic wave energy transmitted through said diskshaped waveguide section and said pumping electromagnetic wave energy supplied to said disk-shaped waveguide section.

8. Claim 6 having means for providing a magnetic field having lines of force substantially perpendicular to the surfaces of said dielectric window to rotate the oscillations of secondary electrons in front of said dielectric window and thereby form a pumping electromagnetic wave having a plane of polarization perpendicular to the plane of polarization of the electromagnetic wave energy transmitted through said output coupling arrangement, whereby secondary electrons emitted from the surface of said dielectric window are driven away from said window due to the low intensity electric field region provided in front of said window which is set up by the standing wave energy derived from both the electromagnetic wave energy transmitted through said disk-shaped waveguide section and said pumping electromagnetic wave energy formed in said disk-shaped waveguide section.

References Cited by the Examiner UNITED STATES PATENTS Re. 23,517 7/1952 Litton 315 -5 2,805,337 9/1957 Dunsmuir 31539.53 X 2,895,110 7/1959 Nelson et a1 3155 2,929,955 3/1960 James 3 l55 3,110,000 11/ 1963 Churchill 333-98 

4. A KLYSTRON INCLUDING AN ELECTRON GUN SECTION, A RADIOFREQUENCY INTERACTION SECTION AND A COLLECTOR SECTION DEFINING AN EVACUATED ENVELOPE, SAID RADIO-FREQUENCY INTERACTION SECTION COMPRISING A PLURALITY OF AXIALLY ALIGNED RESONANT CAVITY PORTIONS, EACH RESONANT CAVITY PORTION CONTAINS ELECTROMAGNETIC WAVE ENERGY AND INCLUDES AN INTERACTION GAP AND CONDUCTIVE METALLIC SURFACES WITHIN THE EVACUATED ENVELOPE, AND AN ELECTROMAGNETIC WAVE TRANSMISSION SYSTEM ASSOCIATED WITH AT LEAST ONE OF SAID RESONANT CAVITY PORTIONS, SAID ELECTROMAGNETIC WAVE TRANSMISSION SYSTEM COMPRISING A WAVEGUIDE SECTION CONTAINING STANDING WAVE ENERGY HAVING ALTERNATE REGIONS OF LOW AND HIGH INTENSITY ELECTRIC FIELDS, SAID WAVEGUIDE SECTION BEING PROVIDED WITH A DIELECTRIC WINDOW TRANSPARENT TO ELECTROMAGNETIC WAVE ENERGY MOUNTED THEREIN, SAID WINDOW BEING LOCATED IN THE LOW INTENSITY ELECTRIC FIELD REGION WITHIN SAID ELECTROMAGNETIC WAVE TRANSMISSION SYSTEM BETWEEN THE HIGH INTENSITY ELECTRIC FIELD REGION IN SAID WAVEGUIDE SECTION AND SAID ASSOCIATED RESONANT CAVITY. 